Magnetic flowmeters (or mag meters) measure flow by application of Faraday's Law, an electromagnetic effect. The magnetic flowmeter energizes one or more coils by passing an excitation current through field windings which generates a magnetic field across an electrically isolated, conductive process fluid flow. An electromotive force (EMF) is generated by the flowing process fluid crossing through the magnetic field. This induced voltage (potential) both across the fluid and with respect to the rest of the process fluid can readily be measured by one or more conductive electrodes that contact the flowing process fluid. The volumetric flow is proportional to the flow velocity and the cross-sectional area of the flowtube. The flow velocity is directly proportional to the electrode voltage potential (EV), which is directly proportional to the induced magnetic field strength (B). The induced magnetic field strength is assumed to be proportional to the applied magnetic field (H), which is directly linked to the magnitude of the excitation current. Thus, a direct correlation is provided between the measured electrode voltage potential and indicated volumetric flow.
Magnetic flowmeters are useful in a variety of conductive and semi-conductive fluid flow measurement environments. In particular, the flow of water-based fluids, ionic solutions and other conducting fluids can all be measured using magnetic flowmeters. Thus, magnetic flowmeters can be found in water treatment facilities, beverage and hygienic food production, chemical processing, high purity pharmaceutical manufacturing, as well as hazardous and coercive processing facilities.
In order to operate effectively, the electrical conductivity of the path extending between the electrodes of the magnetic flowmeter must be relatively high. The overall resistance of this path is determined by that of the fluid itself in series with the interface resistance across the fluid and the surfaces of the electrodes. When magnetic flowmeters measure relatively dirty fluids, such as fluids containing oils or sludge, the contaminants may occasionally coat and adhere to the active surfaces of the electrodes. This coating can change the electrical characteristics of the electrodes and, as the coatings build up, generate an interface resistance that causes measurement errors.
It is known to use ultrasonic cleaning on the electrodes of magnetic flowmeters, particularly when the magnetic flowmeters are used for metering slow-moving liquids which have a tendency to coat the electrodes. The ultrasonic cleaning may, for example, consist of ultrasonic transducers in the form of piezoelectric discs that are positioned in or on the electrode bodies and that are energized periodically by an ultrasonic frequency generator.